1. Field of the Invention
The present invention is related to photoionization detectors for detecting the presence of chemical compounds in a fluid and more particularly to a photoionization detector that comprises multiple ionization cells.
2. Description of the Background Art
Photoionization detectors (PIDs) are conventionally used to detect the presence of chemical compounds in air. When a molecule is exposed to high-energy photons of the appropriate photon energy, the molecule will become ionized. A pair of electrodes are arranged to be exposed to the ionized molecule such that the electrodes are electrically insulated from each other and are maintained at a voltage differential from each other. The ion is repelled from the higher voltage electrode and attracted to the lower voltage electrode. When a fluid stream is exposed to the high-energy photons, and a number of ions are produced, a measurable current is generated.
Conventionally, a PID that is used in an instrument for the purpose of detecting chemical compounds in air includes; an ultraviolet (UV) radiation source as a source for high energy photons and associated electronic circuitry for driving the UV radiation source; an ionization cell into which the output of the UV radiation source is directed, a means for a sample of gas to enter and exit the ionization cell; and two or more electrodes electrically insulated from each other and held at a voltage differential and associated electronic circuitry to maintain the voltage differential, the electrodes having a size, shape, and orientation to effectively produce a current when exposed to ions.
In a classical PID design, a glass discharge UV lamp is used to produce high-energy photons. Typically, the lamp is constructed of a sealed glass volume filled with a gas such as helium, argon, krypton, or xenon with a window made from a material that is highly transmissive of UV radiation such as magnesium fluoride, lithium fluoride, barium fluoride, strontium fluoride, calcium fluoride or sapphire and the glass volume also contains two electrodes. By maintaining the electrodes at a voltage differential, the gas in the glass volume is momentarily excited. The excited gas then returns to the ground state and in doing so emits photons. U.S. Pat. No. 4,398,152 to Leveson describes a gas discharge UV lamp that eliminates the need for electrodes in the glass volume and excites the gas in the glass volume by inductively coupled radio frequency and produced UV radiation that is uniform across the cross-section of the UV lamp. The glass volume is placed in a holder made of polytetrafluoroethylene around which a coupling inductor is wound and connected at one end to an oscillator circuit to generate radio frequency.
The electrodes can have a variety of designs, including, a concentric format with one electrode in the middle of a cylindrically shaped electrode, two disc shaped electrodes in parallel spaced apart with an electric insulator, two thin rods oriented in parallel. The electrodes must be spaced appropriately. Improperly spaced electrodes, either too closely spaced or too widely spaced, will have a negative impact on the overall sensitivity of the detector. Specifically, in the case of too closely spaced electrodes, the electric field is smaller and fewer ions are exposed to the electric field; in the case of too widely spaced electrodes, ions have a greater chance of colliding with a free electron and recombining and the electric field is weaker and would not as strongly attract the ions.
Typically PIDs are designed with small internal volumes and volumes that are continually swept with gas to achieve good analytical performance. Unduly large internal volumes allow the sample to diffuse and generate results that are less accurate. Volumes that are poorly swept, such as holes or tubes that have no exit, or unnecessary changes in cross-sectional areas of the flowpath also allow the sample to diffuse and can allow the sample to reside in the poorly swept volume even after an analysis is complete, and such residual sample can then contaminate a subsequent analysis.
Typically PIDs are made from materials that have high chemical inertness so as to minimize the interaction of the PID with the sample and potentially contaminate the sample and to minimize adsorption of gas into the material to be potentially later released into subsequent analyses.
The sample can be introduced into the ionization cell in a number of ways, including for example: by placing a pump downstream of the PID, the vacuum generated by the pump causing gas to flow through the PID in which case the PID is somewhat evacuated; by placing a pump upstream of the PID, the pressure generated by the output of the pump causing gas to flow through the PID in which case the PID is somewhat pressurized; an injection of sample into the PID by syringe or other means; by injection of the sample into a carrier gas flow stream and passing the sample and carrier gas through a chromatographic column and then through the PID in which case the ionization cell is somewhat pressurized.
In any case, care must be taken to adequately seal the PID from its ambient environment so as not to unduly allow sample or carrier gas to leak out of the PID prior to ionization in the case of a pressurized system, and so as not to allow gases from the ambient environment to be introduced into the ionization cell and potentially contaminate the sample in the ionization cell.
A PID alone typically cannot provide a user with information to distinguish the specific molecules and concentrations of a number of types of molecules that are in a sample. The UV source will emit photons of a specific energy and any molecules that have an ionization energy lower than the energy of the photons will become ionized. If there is a single type of molecule present in the sample that has an ionization energy lower than the energy of the photon, a current will be generated and if compared against a known reference of the same type of molecule, a concentration of the molecule in the sample can be determined. However, if there is more than one type of molecule that is thus ionized, the user will be unable to discern the molecules that are present, the concentrations that are present, nor the number of different types of molecules present.
This drawback of PIDs is typically addressed by passing the fluid to be measured through a chromatographic column prior to introduction into the ionization cell. The various chemical compounds that are in the fluid will be separated from each other in the chromatographic column and will be introduced into the PID individually. The concentrations of each individual chemical compound can then be measured, and in many cases the chemical compounds can be identified based on the timing at which the chemical compounds are eluted from the chromatographic column.
Though it is advantageous to use a chromatographic column to introduce a fluid to the PID, there is a significant delay between the time a fluid sample is collected and the time the various chemical compounds are eluted from the chromatographic column to be measured by the PID. Conversely, if no chromatographic column is used, there is relatively no delay between the time a fluid sample is collected and the time it is introduced into the PID. There exists many applications that require a specific analysis that identifies and quantifies the chemicals in a fluid sample and there exists many applications that require minimal time delay. Also, analyses that require identification and quantification of the chemical compounds tend to also require the measurement of very low concentrations of the chemical compounds and thus require greater sensitivity of the detector. Analyses that require minimal time delay and do not require identification and quantification of chemical compounds often require the measurement of relatively high concentrations of the chemical compounds and thus require a detector capable of measuring a broad range of concentrations of chemical compounds. Though it is advantageous for a chemical compound detection instrument to allow for both types of operation, existing PIDs cannot reconcile the conflicting requirements of the variety of applications.